1. Field of the Invention
The present invention relates to a gallium nitride based semiconductor light emitting device.
2. Description of the Related Art
A gallium nitride (hereinafter referred to as GaN) based semiconductor laser device using a GaN based compound is employed in an optical pickup device for an optical disk as a short wavelength semiconductor laser device for a wavelength range of about 300 to 500 nm.
As shown in FIG. 1, a conventional GaN based semiconductor laser device is made of a multilayer structure formed by providing semiconductor single crystal layers each composed of a GaN based compound on a sapphire substrate 1. The conventional example shown in FIG. 1 is a multilayer structure in which an underlying layer 2, an n-type cladding layer 3, an n-type guiding layer 4, an active layer 5, a p-type guiding layer 6, a p-type cladding layer 7, and a p-type contact layer 8 are deposited on the sapphire substrate 1. In this case, each of the layers is formed of a GaN based compound of which a component ratio is expressed as follows.
(AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61)
Each of the layers is properly doped with a dopant so as to make the layer as an n-type semiconductor or a p-type semiconductor. Further, on the underlying layer 2 and the p-type contact layer 8, there are disposed an n-side electrode 9 and a p-side electrode 10. Furthermore, the p-type cladding layer 7 and the p-type contact layer 8 are provided with a ridge so as to improve the light emission efficiency as a light emitting device. Because by using the ridge structure, a current flow and optical field are concentrated at a part thereof.
In the above arrangement, if a potential is applied between the n-side electrode 9 and the p-side electrode 10, an electron and a hole are recombined with each other in the active layer 5 to emit a light beam. The light beam is propagated through a waveguide formed by the n-type guiding layer 4 and the p-type guiding layer 6 (hereinafter the n-type guiding layer 4, the active layer 5 and the p-type guiding layer 6 are referred to as a waveguide layer).
Incidentally, it is difficult for the GaN based compound to be greatly changed in its refractive index even if a third element is doped therein. As shown in FIG. 2, if a difference in refractive index between the n-type cladding layer 3 and the n-type guiding layer 4 is small, a main lobe 20 of light beams emitted from the active layer 5 can be propagated through the guiding layer. However, side lobes 21, 22, 23 leak from the guiding layer 4 through the n-type cladding layer 3 to the underlying layer 2. For this reason, a far-field distribution of a laser light beam emitted from the device is deviated from a Gaussian distribution. That is, undesirable large magnitude of ripples are observed in the far-field distribution.
In this case, the n-type cladding layer 3 can be designed to have a sufficient thickness so as to prevent the leakage of the light to the underlying layer 2. However, if the n-type cladding layer 3 is made too thick, then the manufacturing cost thereof will unfavorably increase.
Therefore, an object of the present invention is to provide a GaN based semiconductor laser device which can prevent the leakage of the light, which emits from the active layer, through the cladding layer to the underlying layer without making the cladding layer excessively thick.
According to the present invention, there is provided a gallium nitride based semiconductor light emitting device having a multilayer structure obtained by depositing at least an underlying layer, an n-type cladding layer, a waveguide layer containing an active layer, and a p-type cladding layer in this order on a substrate, wherein if the n-type cladding layer, the waveguide layer and the p-type cladding layer are collectively defined as a first three-layer waveguide path and the sapphire substrate, the underlying layer and the n-type cladding layer collectively defined as a second three-layer waveguide path, then effective refractive indices of light propagating through the first and second three-layer waveguide paths are set to be different from each other.
According to the above arrangement of the device, the effective refractive index of light propagating through the first three-layer waveguide path is different from the effective refractive index of light propagating through the second three-layer waveguide path. Therefore, it becomes possible to prevent the leakage of the light, which emits from the active layer, through the cladding layer to the underlying layer due to the mode coupling.